Health organizations are emphasizing that myalgic encephalomyelitis/chronic fatigue syndrome is a serious long-term illness, not a psychological disorder, and that standard forms of exercise do not help. Instead, they're acknowledging that exercise can make the disease much worse unless doctors and patients are very careful.The Centers for Diseases Control and Prevention has already revised its patient guidelines on ME/CFS and is currently revising the ones for physicians.The action by the federal agency is a significant shift in the years-long controversy over diagnosis and management of the disease, which is characterized by months of incapacitating fatigue, joint pain and cognitive problems.And last week, the National Health Services in the U.K. said it would reassess its guidelines for ME/CFS recommending exercise, after 20 members of Parliament signed a petition urging a review.Doctors and patient advocates say the update by the CDC is a major victory for people with ME/CFS, and that a revision of British guidelines is sorely needed. They are hoping other major medical sites will follow suit and revise their information on the disease, which is thought to affect at least a million Americans.Many websites have outdated information that perpetuates a key misperception about CFS: "That you can exercise your way out of this illness," says Dr. Nancy Klimas of Nova Southeastern University in Miami, who has served on the ME/CFS advisory committee for the Department of Health and Human Services and has treated people with ME/CFS for 30 years. "That's just not true. You can exercise, but you have to be extremely cautious. And it will not cure you."Even just a small amount of exercise or simple exertion can make some people much sicker, Klimas says."If a doctor sends a patient to the gym to do endurance exercise, that's going to be a disaster," Klimas says. "Doctors do that all the time, and these patients get much, much worse."In particular, she says, the British guidelines, set by the National Institute for Health Care and Excellence, have been harmful for patients."They are rooted in a very controversial study, which suggests people do a much more vigorous and much more dangerous form of exercise," Klimas says. "Those guidelines have hurt a lot of people. And they were implemented all around the world."The controversial recommendations on exercise can be tracked back to an influential but erroneous paper published in 1989, in which British doctors offered a "new approach" for treating the disease.In the paper, the doctors largely dismissed the disease as psychological. They said the pain patients felt after exercising was basically in patients' heads. And if they exercised long enough, these symptoms would subside.So to treat the disease, the authors presented two recommendations: cognitive behavioral therapy combined with a gradual increase in exercise — even if symptoms started to get worse. The therapy was supposed to help people get over "maladaptive" avoidance of exercise after the initial illness. "For example, if the patient admitted to thinking 'I feel tired, I must have done too much'," the authors wrote, "One might ask the patient to look for alternative explanations, such as 'I may be tired because I haven't being doing much lately.' "Jump ahead 30 years, and what is known about ME/CFS has greatly evolved, says biologist Mauren Hanson, who studies ME/CFS at Cornell University.Doctors now know that the disease makes people incredibly sensitive to exertion. "There's some level of exercise that makes your symptoms get worse," Hanson says.In particular, anaerobic exercise seems to cause the most problems, Hansen says. When your body doesn't have enough oxygen to support exercise, lactic acid starts to build up in muscles."This is when you start feeling out of breath," Hanson says. "There are some expert ME/CFS physicians who have had success with advising patients what heart rate their anaerobic metabolism typically kicks in," Hanson explains, "And then they advise those patients to keep their heart rate below that level."The problem is, this threshold is different for each person. So blanket recommendations for general exercise programs can be extremely harmful. "I've heard many anecdotal cases of individuals who found themselves either bedbound or in wheelchairs after they started exercise programs advised by their physicians," Hanson says.But the medical community has been slow to come up to speed on ME/CFS, Hanson says. Many doctors aren't aware of the dangers with exercising. Some still have the misperception the disease is psychological. And you can still find the two original treatment strategies promoted on major medical websites, such as the Mayo Clinic and WebMD.The CDC's information about ME/CFS has evolved over the past few decades. Back in 2006, their webpages recommended specific types of exercises and even suggested the number of reps to do. Some patients followed instructions like this and saw their symptoms get worse."I did exactly what the CDC told me," says Julie Rehmeyer, a science journalist in Santa Fe, N.M., who wrote a book about her experience with ME/CFS. "I started with a low level of exercise and built up slowly. But if I pushed when I wasn't feeling very good, I found that I would lose the entire next day. I wouldn't be able to walk, get of bed — even move."This past summer, the agency removed the last remnants of the two controversial recommendations. In particular, the agency no longer lists cognitive behavioral therapy as management tool for ME/CFS. And it removed the phrase "graded exercise," which to many doctors in the field referred to the controversial strategy promoted by the NHS in England."We had heard from patients that doctors were using this [guideline] as an excuse to tell patients, 'you must do more and more exercise,' and that's not what it was intended to reflect," says CDC spokesperson Jennifer McQuiston. "We do not want to cause harm."The new guidelines emphasize the need for doctor supervision when it comes to exercise programs for people with ME/CFS, McQuiston says. And doctors need to understand that this program needs to be tailored specifically for each person."We're attempting to caution health care providers, who may be looking at our website: When managing a patient with ME/CFS, it can't be a one-size-fits-all program." Copyright 2017 NPR. To see more, visit http://www.npr.org/.